The invention relates to a method and apparatus for an evacuated glazing element and, more specifically, to an evacuated glazing element including a spacer disposed between adjacent glass panes.
Many existing insulated glazing elements (e.g., vacuum-insulated glass units or assemblies) include two or more glass panes that are separated from each other by a space. Some existing glass panel assemblies incorporate small spacers that are positioned between the glass panes by small spacers, and the space is sealed along the perimeter edge with a strip that is bonded to the outermost glass panes to form a transparent envelope that encloses an evacuated space.
A temperature differential across the glass panel assembly can significantly impact the structure of the assembly and, in some cases, cause the assembly to fail. More specifically, the temperature of the exterior pane typically approaches the outside air temperature (contracting when exposed to cold, expanding when exposed to heat). The interior pane typically remains at a relatively constant temperature that is consistent with the inside air temperature (e.g., in a building). Movement of the exterior pane (i.e. contraction or expansion) relative to the interior pane is known as “differential pane movement.”
Spacers are used to maintain spacing between glass panes while minimizing heat transfer across the space between panes. Some conventional spacers take the form of cylindrical metal pillars that are typically located in a square array, with the circular faces of each pillar in opposite contact with the respective glass panes (i.e. the spacers act as support columns). The spacers often are shaped with a diameter that is larger than the height to avoid overturning in response to a friction force. With these spacers, evacuation of the glass pane envelope (i.e. the space between the panes) induces a significant force (e.g., 2000 pounds) on each square foot of the glass panes. This translates to approximately a 50,000 psi contact stress on each pillar face, which creates a large static friction force between the pillar and the pane. The pressure also results in bending, or “doming,” of the glass pane over each pillar, forming a pattern suggestive of quilted fabric.
During differential pane movement caused by changes in outside air temperature, the high static friction force, exacerbated by the doming of the glass over each pillar, resists slippage so much that the glazing unit can undesirably bow into (or out of) the building. Slippage of the panes over the flat faces of the pillars (when the static friction resistance to movement is overcome) can also cause significant noise and undesirable scratches.
Other spacers have a spherical shape to provide rolling contact between the glass panes and the spacers. However, these spherical spacers have a very small point loading due to their shape and the tangential contact with the panes. This small point loading typically causes damage to the glass due to indentations in the glass surface that is analogous to a broken ice pattern caused by dropping a bowling ball onto the surface of a frozen pool of water. Although the number of spherical spacers can be increased to lower the contact stresses below the threshold for indentation, the large quantity can become visible and the inherent point contact between a sphere and a plane defined by the surface of the glass still typically damages the glass.